This invention relates to methods for evaluating parameters related to the elastic properties of subsurface earth formations, in particular the velocity and direction of propagation of compressional (P) and shear (S) wave components of acoustic energy propagating through the formations. The invention also provides a way of separating the individual waveforms of such components.
Seismic investigation of earth formations is a well-established means of deriving information about sub-surface geological conditions, and is a valuable tool in the search for natural resources such as oil, gas and ores. In such an investigation a seismic wave is propagated through the earth formation to be investigated, by means of a source such as an explosion or a mechanical vibrator. One or more acoustic detectors spaced from the source are used to sense the propagating seismic wave as it passes different locations and comparison of waves detected at these different locations enables information about the formation to be inferred. In one form of seismic investigation the source and/or the detectors are placed in deep boreholes drilled into the earth formation, enabling a vertical seismic profile (VSP) or well-to-well profile to be obtained. The detectors are typically 3-axis geophones which are placed in firm contact with the wall of the borehole and which can produce electrical signals indicative of movement of the formation along each of three orthogonal axes.
The waveform signals thus derived can be analyzed in a variety of ways to obtain information about subsurface conditions. Recently interest has been increasing in the possibility of deriving individual values for the compressional and shear propagation characteristics of formations. Whereas fluids can only support compressional (P) sonic waves, in which the particle motion comprising the wave is parallel to the direction of wave propagation, solids can also sustain shear (S) waves, in which the direction of particle motion is transverse to the direction of wave propagation. The velocities of P and S waves are two independent parameters related to the elastic properties of formations and are of interest for various reasons. Thus one useful indicator of lithology type is the ratio of P and S wave velocities. Imaging of reflected compressional and shear waves in surface seismics or vertical seismic profiles can provide valuable information, but requires values of P and S wave velocity as a function of depth.
Techniques are already known for separating and extracting P wave components and S wave components in the waveforms obtained during VSP measurements. See for example U.S. Pat. No. 4,648,039 to Devaney et al, assigned to the assignee of this application. However, these techniques require knowledge of the velocity of propagation of the P and S waves in the formation. Determination of P wave velocity is possible by having a skilled user pick waveform events which appear characteristic of the arrival of P waves at the detectors. The time offset between corresponding arrivals at detectors having a known spacing is related to the velocity.
However, picking arrivals accurately is difficult and time-consuming even in the case of P wave events, which are generally the first significant signals to appear on a seismic recording. S wave events usually occur later, in the midst of other events such as P wave reflections and mode conversions. Thus picking S wave arrivals can be very difficult and is sometimes impossible.
Moreover, if the seismic energy is not propagating vertically, as is the case in VSP measurments with an offset source, it is necessary to determine the angle of incidence of the waves at the detectors as well as the time offset between between detectors in order to obtain correct values for the velocity of propagation. Although the angle of incidence can be determined to some extent from the particle motion of the waves within time windows specified by an experienced user, this operation again is difficult, time-consuming and subject to error, as well as requiring the intervention of a skilled user.
Accordingly there is a need for a method of determined velocities of both P and S wave propagation which is fast and reliable, which reduces or eliminates the need for user interaction and skill, and which avoids the need to identify specific events such as arrivals.
It is an object of this invention to provide a method of determining velocity of P wave and S wave propagation in a formation from three (or two) component seismic measurements such as VSP measurments, which reduces or avoids the need for identification of arrivals.
It is another object of this invention to provide a method of determining direction of P wave and S wave propagation in a formation from three (or two) component seismic measurements such as VSP measurements.
It is a further object of this invention to provide a method of separating P and S waveform components in seismic measurements such as VSP measurements.